The open-framework structure of KSbGe3O9 flux-grown crystals investigated by X-ray diffraction, vibrational spectroscopy, and DFT calculations

We report on the preparation and the X-ray crystal structure of colorless KSbGe3O9, its vibrational properties (Raman and infrared studies), and density functional theory (DFT) calculations. KSbGe3O9, grown by the high-temperature flux method from K2Mo4O13 flux, is thermally stable at least up to 1200 ​°C and is isostructural to the benitoite BaTiSi3O9 (space group P6¯c2 (no. 188)). The hexagonal unit cell contains two formula units and the structure was refined to R1 ​= ​0.0324 from single-crystal X-ray diffraction data. KSbGe3O9 is characterized with only one crystallographically independent Ge atom involved in three-member units [Ge3O9]6- of regular germanate tetrahedra. The K+ ions are located in channels and, like SbV, are octahedrally surrounded by oxygen atoms. The KSbGe3O9 local structure and the planarity of Ge3O3 rings are also studied by a room-temperature vibrational investigation using non-polarized infrared and Raman spectroscopy. Both the infrared and Raman phonon modes have been assigned from the agreement observed between our experimental data and the corresponding DFT ones. In particular, two E’ (TO) modes (both IR and Raman active) characterize the planarity of the Ge3O3 ring in the ab plane. First-principle calculations have identified two normal modes, both IR and Raman active, due to motions of the oxygen atoms forming the planar Ge3O3 central ring in the ab plane of KSbGe3O9. [Display omitted] •Colorless and transparent crystals chemically and structurally stable up to 1200 ​°C.•Regular SbO6 octahedra and [Ge3O9]6- units form 3-D channels containing K+ ions.•KSbGe3O9 with planar Ge3O3 rings belongs to the benitoite-type structure.•Two E′ (TO) modes characterize the planarity of the Ge3O3 ring within the ab plane.